fix: prevent empty baseURL/region string in Bedrock provider config (#14425)

### What this PR does

Before this PR: When a user configures the built-in AWS Bedrock provider
(leaving **API Host** blank, which is the default), every request fails
immediately with `failed to fetch`. This happens regardless of whether
API Key or IAM authentication is used, and regardless of whether the
Region and credentials are correctly filled in.

After this PR: The SDK correctly constructs the endpoint URL from the
configured Region automatically (e.g.
`https://bedrock-runtime.us-east-1.amazonaws.com`), and requests should
succeed normally.

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Fixes #14423

### Why we need it and why it was done in this way

`buildBedrockConfig()` spreads `ctx.baseConfig` (which contains
`baseURL: ""` when the user leaves API Host blank) directly into the
`providerSettings` passed to `createAmazonBedrock()`.

Inside `@ai-sdk/amazon-bedrock`, the base URL selection logic uses a
loose `!= null` check:

```/dev/null/sdk-snippet.js#L1-5
// Inside @ai-sdk/amazon-bedrock: bedrock-provider.ts
const getBedrockRuntimeBaseUrl = () => {
  return withoutTrailingSlash(
    options.baseURL != null        // "" != null → true (empty string passes this check!)
      ? options.baseURL            // → uses "" as the base URL
      : `https://bedrock-runtime.${region}.amazonaws.com`
  ) ?? `https://bedrock-runtime.us-east-1.amazonaws.com`;
  // The ?? fallback also never triggers: withoutTrailingSlash("") returns ""
};
```

Because `"" != null` is `true`, an empty string is treated as a valid,
intentional base URL. Every request then goes to
`""/model/{modelId}/converse` — an invalid URL — causing the `failed to
fetch` failure.

The fix converts both `baseURL` and `region` from empty string to
`undefined` before passing them to the SDK. `undefined != null` is
`false`, so the SDK correctly falls back to constructing the proper
endpoint from the region. The same guard on `region` prevents a
secondary issue where an empty region would produce a malformed URL like
`https://bedrock-runtime..amazonaws.com`.

The following tradeoffs were made:

- Using `|| undefined` (falsy check) rather than a strict `=== ""`
check, so any other whitespace-only or otherwise empty value is also
handled safely.
- The fix is applied only inside `buildBedrockConfig()` and does not
affect any other provider.

The following alternatives were considered:

- Patching the `@ai-sdk/amazon-bedrock` package — rejected because the
SDK's behavior is internally consistent.

Links to places where the discussion took place: <!-- optional: slack,
other GH issue, mailinglist, ... -->

### Breaking changes

None.

<!-- optional -->

If this PR introduces breaking changes, please describe the changes and
the impact on users.

### Special notes for your reviewer

The changed function is `buildBedrockConfig()` in
`src/renderer/src/aiCore/provider/providerConfig.ts`. The diff is
minimal: one variable extraction for `baseURL`, one `|| undefined` guard
on `region`, and the propagation of `baseURL` into both return paths.

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### Release note

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```release-note
Fixed AWS Bedrock requests failing with "failed to fetch" when API Host is left blank (the default). The SDK endpoint should now be correctly derived from the configured Region.
```

---------

Signed-off-by: Tsudrat <tsudrat@proton.me>
Co-authored-by: Tsudrat <tsudrat@proton.me>
Co-authored-by: 亢奋猫 <kangfenmao@qq.com>

src/renderer/src/aiCore/provider/providerConfig.ts

@@ -209,17 +209,20 @@ function buildOllamaConfig(ctx: BuilderContext): ProviderConfig<'ollama'> {
 
 function buildBedrockConfig(ctx: BuilderContext): ProviderConfig<'bedrock'> {
   const authType = getAwsBedrockAuthType()
-  const region = getAwsBedrockRegion()
+  const region = getAwsBedrockRegion().trim() || undefined
 
   const base = { providerId: 'bedrock' as const, endpoint: ctx.endpoint }
 
+  const baseURL = ctx.baseConfig.baseURL || undefined
+
   if (authType === 'apiKey') {
-    return { ...base, providerSettings: { ...ctx.baseConfig, region, apiKey: getAwsBedrockApiKey() } }
+    return { ...base, providerSettings: { ...ctx.baseConfig, baseURL, region, apiKey: getAwsBedrockApiKey() } }
   }
   return {
     ...base,
     providerSettings: {
       ...ctx.baseConfig,
+      baseURL,
       region,
       accessKeyId: getAwsBedrockAccessKeyId(),
       secretAccessKey: getAwsBedrockSecretAccessKey()